gcc/tree-phinodes.c

   1 /* Generic routines for manipulating PHIs
   2    Copyright (C) 2003-2015 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   6 GCC is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 3, or (at your option)
   9 any later version.
  10
  11 GCC is distributed in the hope that it will be useful,
  12 but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 GNU General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #include "config.h"
  21 #include "system.h"
  22 #include "coretypes.h"
  23 #include "tm.h"
  24 #include "input.h"
  25 #include "alias.h"
  26 #include "symtab.h"
  27 #include "tree.h"
  28 #include "fold-const.h"
  29 #include "predict.h"
  30 #include "hard-reg-set.h"
  31 #include "input.h"
  32 #include "function.h"
  33 #include "basic-block.h"
  34 #include "tree-ssa-alias.h"
  35 #include "internal-fn.h"
  36 #include "gimple-expr.h"
  37 #include "is-a.h"
  38 #include "gimple.h"
  39 #include "gimple-iterator.h"
  40 #include "gimple-ssa.h"
  41 #include "tree-phinodes.h"
  42 #include "ssa-iterators.h"
  43 #include "stringpool.h"
  44 #include "tree-ssanames.h"
  45 #include "tree-ssa.h"
  46 #include "diagnostic-core.h"
  47
  48 /* Rewriting a function into SSA form can create a huge number of PHIs
  49    many of which may be thrown away shortly after their creation if jumps
  50    were threaded through PHI nodes.
  51
  52    While our garbage collection mechanisms will handle this situation, it
  53    is extremely wasteful to create nodes and throw them away, especially
  54    when the nodes can be reused.
  55
  56    For PR 8361, we can significantly reduce the number of nodes allocated
  57    and thus the total amount of memory allocated by managing PHIs a
  58    little.  This additionally helps reduce the amount of work done by the
  59    garbage collector.  Similar results have been seen on a wider variety
  60    of tests (such as the compiler itself).
  61
  62    PHI nodes have different sizes, so we can't have a single list of all
  63    the PHI nodes as it would be too expensive to walk down that list to
  64    find a PHI of a suitable size.
  65
  66    Instead we have an array of lists of free PHI nodes.  The array is
  67    indexed by the number of PHI alternatives that PHI node can hold.
  68    Except for the last array member, which holds all remaining PHI
  69    nodes.
  70
  71    So to find a free PHI node, we compute its index into the free PHI
  72    node array and see if there are any elements with an exact match.
  73    If so, then we are done.  Otherwise, we test the next larger size
  74    up and continue until we are in the last array element.
  75
  76    We do not actually walk members of the last array element.  While it
  77    might allow us to pick up a few reusable PHI nodes, it could potentially
  78    be very expensive if the program has released a bunch of large PHI nodes,
  79    but keeps asking for even larger PHI nodes.  Experiments have shown that
  80    walking the elements of the last array entry would result in finding less
  81    than .1% additional reusable PHI nodes.
  82
  83    Note that we can never have less than two PHI argument slots.  Thus,
  84    the -2 on all the calculations below.  */
  85
  86 #define NUM_BUCKETS 10
  87 static GTY ((deletable (""))) vec<gimple, va_gc> *free_phinodes[NUM_BUCKETS - 2];
  88 static unsigned long free_phinode_count;
  89
  90 static int ideal_phi_node_len (int);
  91
  92 unsigned int phi_nodes_reused;
  93 unsigned int phi_nodes_created;
  94
  95 /* Dump some simple statistics regarding the re-use of PHI nodes.  */
  96
  97 void
  98 phinodes_print_statistics (void)
  99 {
 100   fprintf (stderr, "PHI nodes allocated: %u\n", phi_nodes_created);
 101   fprintf (stderr, "PHI nodes reused: %u\n", phi_nodes_reused);
 102 }
 103
 104 /* Allocate a PHI node with at least LEN arguments.  If the free list
 105    happens to contain a PHI node with LEN arguments or more, return
 106    that one.  */
 107
 108 static inline gphi *
 109 allocate_phi_node (size_t len)
 110 {
 111   gphi *phi;
 112   size_t bucket = NUM_BUCKETS - 2;
 113   size_t size = sizeof (struct gphi)
 114                 + (len - 1) * sizeof (struct phi_arg_d);
 115
 116   if (free_phinode_count)
 117     for (bucket = len - 2; bucket < NUM_BUCKETS - 2; bucket++)
 118       if (free_phinodes[bucket])
 119         break;
 120
 121   /* If our free list has an element, then use it.  */
 122   if (bucket < NUM_BUCKETS - 2
 123       && gimple_phi_capacity ((*free_phinodes[bucket])[0]) >= len)
 124     {
 125       free_phinode_count--;
 126       phi = as_a <gphi *> (free_phinodes[bucket]->pop ());
 127       if (free_phinodes[bucket]->is_empty ())
 128         vec_free (free_phinodes[bucket]);
 129       if (GATHER_STATISTICS)
 130         phi_nodes_reused++;
 131     }
 132   else
 133     {
 134       phi = static_cast <gphi *> (ggc_internal_alloc (size));
 135       if (GATHER_STATISTICS)
 136         {
 137           enum gimple_alloc_kind kind = gimple_alloc_kind (GIMPLE_PHI);
 138           phi_nodes_created++;
 139           gimple_alloc_counts[(int) kind]++;
 140           gimple_alloc_sizes[(int) kind] += size;
 141         }
 142     }
 143
 144   return phi;
 145 }
 146
 147 /* Given LEN, the original number of requested PHI arguments, return
 148    a new, "ideal" length for the PHI node.  The "ideal" length rounds
 149    the total size of the PHI node up to the next power of two bytes.
 150
 151    Rounding up will not result in wasting any memory since the size request
 152    will be rounded up by the GC system anyway.  [ Note this is not entirely
 153    true since the original length might have fit on one of the special
 154    GC pages. ]  By rounding up, we may avoid the need to reallocate the
 155    PHI node later if we increase the number of arguments for the PHI.  */
 156
 157 static int
 158 ideal_phi_node_len (int len)
 159 {
 160   size_t size, new_size;
 161   int log2, new_len;
 162
 163   /* We do not support allocations of less than two PHI argument slots.  */
 164   if (len < 2)
 165     len = 2;
 166
 167   /* Compute the number of bytes of the original request.  */
 168   size = sizeof (struct gphi)
 169          + (len - 1) * sizeof (struct phi_arg_d);
 170
 171   /* Round it up to the next power of two.  */
 172   log2 = ceil_log2 (size);
 173   new_size = 1 << log2;
 174
 175   /* Now compute and return the number of PHI argument slots given an
 176      ideal size allocation.  */
 177   new_len = len + (new_size - size) / sizeof (struct phi_arg_d);
 178   return new_len;
 179 }
 180
 181 /* Return a PHI node with LEN argument slots for variable VAR.  */
 182
 183 static gphi *
 184 make_phi_node (tree var, int len)
 185 {
 186   gphi *phi;
 187   int capacity, i;
 188
 189   capacity = ideal_phi_node_len (len);
 190
 191   phi = allocate_phi_node (capacity);
 192
 193   /* We need to clear the entire PHI node, including the argument
 194      portion, because we represent a "missing PHI argument" by placing
 195      NULL_TREE in PHI_ARG_DEF.  */
 196   memset (phi, 0, (sizeof (struct gphi)
 197                    - sizeof (struct phi_arg_d)
 198                    + sizeof (struct phi_arg_d) * len));
 199   phi->code = GIMPLE_PHI;
 200   gimple_init_singleton (phi);
 201   phi->nargs = len;
 202   phi->capacity = capacity;
 203   if (!var)
 204     ;
 205   else if (TREE_CODE (var) == SSA_NAME)
 206     gimple_phi_set_result (phi, var);
 207   else
 208     gimple_phi_set_result (phi, make_ssa_name (var, phi));
 209
 210   for (i = 0; i < capacity; i++)
 211     {
 212       use_operand_p  imm;
 213
 214       gimple_phi_arg_set_location (phi, i, UNKNOWN_LOCATION);
 215       imm = gimple_phi_arg_imm_use_ptr (phi, i);
 216       imm->use = gimple_phi_arg_def_ptr (phi, i);
 217       imm->prev = NULL;
 218       imm->next = NULL;
 219       imm->loc.stmt = phi;
 220     }
 221
 222   return phi;
 223 }
 224
 225 /* We no longer need PHI, release it so that it may be reused.  */
 226
 227 void
 228 release_phi_node (gimple phi)
 229 {
 230   size_t bucket;
 231   size_t len = gimple_phi_capacity (phi);
 232   size_t x;
 233
 234   for (x = 0; x < gimple_phi_num_args (phi); x++)
 235     {
 236       use_operand_p  imm;
 237       imm = gimple_phi_arg_imm_use_ptr (phi, x);
 238       delink_imm_use (imm);
 239     }
 240
 241   bucket = len > NUM_BUCKETS - 1 ? NUM_BUCKETS - 1 : len;
 242   bucket -= 2;
 243   vec_safe_push (free_phinodes[bucket], phi);
 244   free_phinode_count++;
 245 }
 246
 247
 248 /* Resize an existing PHI node.  The only way is up.  Return the
 249    possibly relocated phi.  */
 250
 251 static gphi *
 252 resize_phi_node (gphi *phi, size_t len)
 253 {
 254   size_t old_size, i;
 255   gphi *new_phi;
 256
 257   gcc_assert (len > gimple_phi_capacity (phi));
 258
 259   /* The garbage collector will not look at the PHI node beyond the
 260      first PHI_NUM_ARGS elements.  Therefore, all we have to copy is a
 261      portion of the PHI node currently in use.  */
 262   old_size = sizeof (struct gphi)
 263              + (gimple_phi_num_args (phi) - 1) * sizeof (struct phi_arg_d);
 264
 265   new_phi = allocate_phi_node (len);
 266
 267   memcpy (new_phi, phi, old_size);
 268
 269   for (i = 0; i < gimple_phi_num_args (new_phi); i++)
 270     {
 271       use_operand_p imm, old_imm;
 272       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 273       old_imm = gimple_phi_arg_imm_use_ptr (phi, i);
 274       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 275       relink_imm_use_stmt (imm, old_imm, new_phi);
 276     }
 277
 278   new_phi->capacity = len;
 279
 280   for (i = gimple_phi_num_args (new_phi); i < len; i++)
 281     {
 282       use_operand_p imm;
 283
 284       gimple_phi_arg_set_location (new_phi, i, UNKNOWN_LOCATION);
 285       imm = gimple_phi_arg_imm_use_ptr (new_phi, i);
 286       imm->use = gimple_phi_arg_def_ptr (new_phi, i);
 287       imm->prev = NULL;
 288       imm->next = NULL;
 289       imm->loc.stmt = new_phi;
 290     }
 291
 292   return new_phi;
 293 }
 294
 295 /* Reserve PHI arguments for a new edge to basic block BB.  */
 296
 297 void
 298 reserve_phi_args_for_new_edge (basic_block bb)
 299 {
 300   size_t len = EDGE_COUNT (bb->preds);
 301   size_t cap = ideal_phi_node_len (len + 4);
 302   gphi_iterator gsi;
 303
 304   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 305     {
 306       gphi *stmt = gsi.phi ();
 307
 308       if (len > gimple_phi_capacity (stmt))
 309         {
 310           gphi *new_phi = resize_phi_node (stmt, cap);
 311
 312           /* The result of the PHI is defined by this PHI node.  */
 313           SSA_NAME_DEF_STMT (gimple_phi_result (new_phi)) = new_phi;
 314           gsi_set_stmt (&gsi, new_phi);
 315
 316           release_phi_node (stmt);
 317           stmt = new_phi;
 318         }
 319
 320       /* We represent a "missing PHI argument" by placing NULL_TREE in
 321          the corresponding slot.  If PHI arguments were added
 322          immediately after an edge is created, this zeroing would not
 323          be necessary, but unfortunately this is not the case.  For
 324          example, the loop optimizer duplicates several basic blocks,
 325          redirects edges, and then fixes up PHI arguments later in
 326          batch.  */
 327       SET_PHI_ARG_DEF (stmt, len - 1, NULL_TREE);
 328       gimple_phi_arg_set_location (stmt, len - 1, UNKNOWN_LOCATION);
 329
 330       stmt->nargs++;
 331     }
 332 }
 333
 334 /* Adds PHI to BB.  */
 335
 336 void
 337 add_phi_node_to_bb (gphi *phi, basic_block bb)
 338 {
 339   gimple_seq seq = phi_nodes (bb);
 340   /* Add the new PHI node to the list of PHI nodes for block BB.  */
 341   if (seq == NULL)
 342     set_phi_nodes (bb, gimple_seq_alloc_with_stmt (phi));
 343   else
 344     {
 345       gimple_seq_add_stmt (&seq, phi);
 346       gcc_assert (seq == phi_nodes (bb));
 347     }
 348
 349   /* Associate BB to the PHI node.  */
 350   gimple_set_bb (phi, bb);
 351
 352 }
 353
 354 /* Create a new PHI node for variable VAR at basic block BB.  */
 355
 356 gphi *
 357 create_phi_node (tree var, basic_block bb)
 358 {
 359   gphi *phi = make_phi_node (var, EDGE_COUNT (bb->preds));
 360
 361   add_phi_node_to_bb (phi, bb);
 362   return phi;
 363 }
 364
 365
 366 /* Add a new argument to PHI node PHI.  DEF is the incoming reaching
 367    definition and E is the edge through which DEF reaches PHI.  The new
 368    argument is added at the end of the argument list.
 369    If PHI has reached its maximum capacity, add a few slots.  In this case,
 370    PHI points to the reallocated phi node when we return.  */
 371
 372 void
 373 add_phi_arg (gphi *phi, tree def, edge e, source_location locus)
 374 {
 375   basic_block bb = e->dest;
 376
 377   gcc_assert (bb == gimple_bb (phi));
 378
 379   /* We resize PHI nodes upon edge creation.  We should always have
 380      enough room at this point.  */
 381   gcc_assert (gimple_phi_num_args (phi) <= gimple_phi_capacity (phi));
 382
 383   /* We resize PHI nodes upon edge creation.  We should always have
 384      enough room at this point.  */
 385   gcc_assert (e->dest_idx < gimple_phi_num_args (phi));
 386
 387   /* Copy propagation needs to know what object occur in abnormal
 388      PHI nodes.  This is a convenient place to record such information.  */
 389   if (e->flags & EDGE_ABNORMAL)
 390     {
 391       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def) = 1;
 392       SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (phi)) = 1;
 393     }
 394
 395   SET_PHI_ARG_DEF (phi, e->dest_idx, def);
 396   gimple_phi_arg_set_location (phi, e->dest_idx, locus);
 397 }
 398
 399
 400 /* Remove the Ith argument from PHI's argument list.  This routine
 401    implements removal by swapping the last alternative with the
 402    alternative we want to delete and then shrinking the vector, which
 403    is consistent with how we remove an edge from the edge vector.  */
 404
 405 static void
 406 remove_phi_arg_num (gphi *phi, int i)
 407 {
 408   int num_elem = gimple_phi_num_args (phi);
 409
 410   gcc_assert (i < num_elem);
 411
 412   /* Delink the item which is being removed.  */
 413   delink_imm_use (gimple_phi_arg_imm_use_ptr (phi, i));
 414
 415   /* If it is not the last element, move the last element
 416      to the element we want to delete, resetting all the links. */
 417   if (i != num_elem - 1)
 418     {
 419       use_operand_p old_p, new_p;
 420       old_p = gimple_phi_arg_imm_use_ptr (phi, num_elem - 1);
 421       new_p = gimple_phi_arg_imm_use_ptr (phi, i);
 422       /* Set use on new node, and link into last element's place.  */
 423       *(new_p->use) = *(old_p->use);
 424       relink_imm_use (new_p, old_p);
 425       /* Move the location as well.  */
 426       gimple_phi_arg_set_location (phi, i,
 427                                    gimple_phi_arg_location (phi, num_elem - 1));
 428     }
 429
 430   /* Shrink the vector and return.  Note that we do not have to clear
 431      PHI_ARG_DEF because the garbage collector will not look at those
 432      elements beyond the first PHI_NUM_ARGS elements of the array.  */
 433   phi->nargs--;
 434 }
 435
 436
 437 /* Remove all PHI arguments associated with edge E.  */
 438
 439 void
 440 remove_phi_args (edge e)
 441 {
 442   gphi_iterator gsi;
 443
 444   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 445     remove_phi_arg_num (gsi.phi (),
 446                         e->dest_idx);
 447 }
 448
 449
 450 /* Remove the PHI node pointed-to by iterator GSI from basic block BB.  After
 451    removal, iterator GSI is updated to point to the next PHI node in the
 452    sequence. If RELEASE_LHS_P is true, the LHS of this PHI node is released
 453    into the free pool of SSA names.  */
 454
 455 void
 456 remove_phi_node (gimple_stmt_iterator *gsi, bool release_lhs_p)
 457 {
 458   gimple phi = gsi_stmt (*gsi);
 459
 460   if (release_lhs_p)
 461     insert_debug_temps_for_defs (gsi);
 462
 463   gsi_remove (gsi, false);
 464
 465   /* If we are deleting the PHI node, then we should release the
 466      SSA_NAME node so that it can be reused.  */
 467   release_phi_node (phi);
 468   if (release_lhs_p)
 469     release_ssa_name (gimple_phi_result (phi));
 470 }
 471
 472 /* Remove all the phi nodes from BB.  */
 473
 474 void
 475 remove_phi_nodes (basic_block bb)
 476 {
 477   gphi_iterator gsi;
 478
 479   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); )
 480     remove_phi_node (&gsi, true);
 481
 482   set_phi_nodes (bb, NULL);
 483 }
 484
 485 /* Given PHI, return its RHS if the PHI is a degenerate, otherwise return
 486    NULL.  */
 487
 488 tree
 489 degenerate_phi_result (gphi *phi)
 490 {
 491   tree lhs = gimple_phi_result (phi);
 492   tree val = NULL;
 493   size_t i;
 494
 495   /* Ignoring arguments which are the same as LHS, if all the remaining
 496      arguments are the same, then the PHI is a degenerate and has the
 497      value of that common argument.  */
 498   for (i = 0; i < gimple_phi_num_args (phi); i++)
 499     {
 500       tree arg = gimple_phi_arg_def (phi, i);
 501
 502       if (arg == lhs)
 503         continue;
 504       else if (!arg)
 505         break;
 506       else if (!val)
 507         val = arg;
 508       else if (arg == val)
 509         continue;
 510       /* We bring in some of operand_equal_p not only to speed things
 511          up, but also to avoid crashing when dereferencing the type of
 512          a released SSA name.  */
 513       else if (TREE_CODE (val) != TREE_CODE (arg)
 514                || TREE_CODE (val) == SSA_NAME
 515                || !operand_equal_p (arg, val, 0))
 516         break;
 517     }
 518   return (i == gimple_phi_num_args (phi) ? val : NULL);
 519 }
 520
 521 /* Set PHI nodes of a basic block BB to SEQ.  */
 522
 523 void
 524 set_phi_nodes (basic_block bb, gimple_seq seq)
 525 {
 526   gimple_stmt_iterator i;
 527
 528   gcc_checking_assert (!(bb->flags & BB_RTL));
 529   bb->il.gimple.phi_nodes = seq;
 530   if (seq)
 531     for (i = gsi_start (seq); !gsi_end_p (i); gsi_next (&i))
 532       gimple_set_bb (gsi_stmt (i), bb);
 533 }
 534
 535 #include "gt-tree-phinodes.h"